Compliant foil fluid film radial bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a bushing, a rotating element such as a rotor or shaft adapted to rotate within the bushing, non-rotating compliant fluid foil members mounted within the bushing and enclosing the rotating element, and non-rotating compliant spring foil members mounted within the bushing underneath the non-rotating compliant fluid foil members. The space between the rotating element and the bushing is filled with fluid (usually air) which envelops the foils. Conventionally, the compliant fluid foil elements are divided into a plurality of individual compliant foils to form a plurality of wedge shaped channels which converge in thickness in the direction of the rotation of the rotor.
The motion of the rotating element applies viscous drag forces to the fluid in the converging wedge channels. This results in increases in fluid pressure, especially near the trailing end of the wedge channels. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases, causing the fluid pressure rise along the channel to increase. Conversely, if the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils causes coulomb damping of any axial or overturning motion of the rotating element of the bearing.
Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the lift-off/touch-down speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements.
Compliant foil fluid film radial bearings typically rely on backing springs to preload the fluid foils against the relatively movable rotating element so as to control foil position/nesting and to establish foil dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the rotor speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high lift-off/touchdown speeds result in significant bearing wear each time the rotor is started or stopped.
Conventional compliant foil fluid film radial bearings operate with extremely small running clearances and moderate, as opposed to low, drag and power consumption. The clearances between the non-rotating fluid foil's converging channel ramp trailing ends and the rotating element are typically less than 100 micro-inches at operating conditions.
While most prior compliant foil fluid film radial bearings utilize a plurality of individual compliant foils and individual spring foils with a cylindrically bored bushing, there are instances where a single compliant foil and a single spring foil have been proposed with a cam shaped or lobed bushing. An example of this type of radial bearing can be found in U.S. Pat. No. 5,427,455 issued Jun. 27, 1995 to Robert W. Bosley, entitled “Compliant Foil Hydrodynamic Fluid Film Radial Bearing”.